Both chloramphenicol and thiamphenicol-induced servere aplastic anemia in bovine calves when administered orally at a dose of 100 mg/kg/day for 10 days. Aplastic anemia induced by thiamphenicol was completely prevented by co-administration of B complex vitamins, Vitamin B12 and folic acid. However, chloramphenicol-induced aplastic anemia was only reduced and not completely prevented by the administration of vitamin mixture indicating a still unknown mechanism is involved in chloramphenicol induced aplastic anemia.